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X-ray_photoelectron_spectroscopy

X-ray photoelectron spectroscopy

X-ray Photoelectron Spectroscopy (XPS) is a quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material. XPS spectra are obtained by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energy (KE) and number of electrons that escape from the top 1 to 10 nm of the material being analyzed. XPS requires ultra-high vacuum (UHV) conditions.

XPS is a surface chemical analysis technique that can be used to analyze the surface chemistry of a material in its "as received" state, or after some treatment such as: fracturing, cutting or scraping in air or UHV to expose the bulk chemistry, ion beam etching to clean off some of the surface contamination, exposure to heat to study the changes due to heating, exposure to reactive gases or solutions, exposure to ion beam implant, exposure to ultraviolet light, for example.

XPS is also known as ESCA, an abbreviation for Electron Spectroscopy for Chemical Analysis.

XPS detects all elements with an atomic number (Z) between those of lithium (Z=3) and lawrencium (Z=103). This limitation means that it cannot detect hydrogen (Z=1) or helium (Z=2).

Detection limits for most of the elements are in the parts per thousand range. Detections limits of parts per million (ppm) are possible, but require special conditions: concentration at top surface or very long collection time (overnight).

XPS is routinely used to analyze inorganic compounds, metal alloys, semiconductors, polymers, elements, catalysts, glasses, ceramics, paints, papers, inks, woods, plant parts, make-up, teeth, bones, medical implants, bio-materials, viscous oils, glues, ion modified materials and many others.

XPS is used to measure:

elemental composition of the surface (1–10 nm usually)
empirical formula of pure materials
elements that contaminate a surface
chemical or electronic state of each element in the surface
uniformity of elemental composition across the top surface (aka, line profiling or mapping)
uniformity of elemental composition as a function of ion beam etching (aka, depth profiling)

XPS can be performed using either a commercially built XPS system, a privately built XPS system or a Synchrotron-based light source combined with a custom designed electron analyzer. Commercial XPS instruments in the year 2005 used either a highly focused 20 to 200 micrometer beam of monochromatic aluminum K-alpha X-rays or a broad 10-30 mm beam of non-monochromatic (achromatic or polychromatic) magnesium X-rays. A few, special design XPS instruments can analyze volatile liquids or gases, materials at low or high temperatures or materials at roughly 1 torr vacuum, but there are relatively few of these types of XPS systems.

Because the energy of a particular X-ray wavelength equals a known quantity, we can determine the electron binding energy (BE) of each of the emitted electrons by using an equation that is based on the work of Ernest Rutherford (1914):

E_binding = E_photon - E_kinetic - Φ

where E_binding is the energy of the electron emitted from one electron configuration within the atom, E_photon is the energy of the X-ray photons being used, E_kinetic is the kinetic energy of the emitted electron as measured by the instrument and Φ is the work function of the spectrometer (not the material).

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History of XPS

In 1887, Heinrich Rudolf Hertz discovered the photoelectric effect. Twenty years later, in 1907, P.D. Innes experimented with a Röntgen tube, Helmholtz coils, a magnetic field hemisphere (electron energy analyzer) and photographic plates to record broad bands of emitted electrons as a function of velocity, in effect recording the first XPS spectrum. Other researchers, Moseley, Rawlinson and Robinson, independently performed various experiments trying to sort out the details in the broad bands. Due to the wars, research on XPS came to a halt. After WWII, Kai Siegbahn and his group in Sweden developed several significant improvements in the equipment and in 1954 recorded the first high energy resolution XPS spectrum of cleaved sodium chloride (NaCl) revealing the potential of XPS. A few years later in 1967, Siegbahn published a comprehensive study on XPS bringing instant recognition of the utility of XPS. In cooperation with Siegbahn, Hewlett-Packard in the USA produced the first commercial monochromatic XPS instrument in 1969. Siegbahn received the Nobel Prize in 1981 to acknowledge his extensive efforts to develop XPS into a useful analytical tool.

Physics of XPS

A typical XPS spectrum is a plot of the number of electrons detected (Y-axis, ordinate) versus the binding energy of the electrons detected (X-axis, abscissa). Each element produces a characteristic set of XPS peaks at characteristic binding energy values that directly identify each element that exist in or on the surface of the material being analyzed. These characteristic peaks correspond to the electron configuration of the electrons within the atoms, e.g., 1s, 2s, 2p, 3s, etc. The number of detected electrons in each of the characteristic peaks is directly related to the amount of element within the area (volume) irradiated. To generate atomic percentage values, each raw XPS signal must be corrected by dividing its signal intensity (number of electrons detected) by a "relative sensitivity factor" (RSF) and normalized over all of the elements detected.

To count the number of electrons at each KE value, with the minimum of error, XPS must be performed under ultra-high vacuum (UHV) conditions because electron counting detectors in XPS instruments are typically one meter away from the material irradiated with X-rays.

It is important to note that XPS detects only those electrons that have actually escaped into the vacuum of the instrument. The photo-emitted electrons that have escaped into the vacuum of the instrument are those that originated from within the top 10 to 12 nm of the material. All of the deeper photo-emitted electrons, which were generated as the X-rays penetrated 1–5 micrometers of the material, are either recaptured or trapped in various excited states within the material. For most applications, it is, in effect, a non-destructive technique that measures the surface chemistry of any material.

Components of an XPS system

The main components of an XPS system include:

A source of X-rays
An ultra-high vacuum (UHV) stainless steel chamber with UHV pumps
An electron collection lens
An electron energy analyzer
Mu-metal magnetic field shielding
An electron detector system
A moderate vacuum sample introduction chamber
Sample mounts
A sample stage
A set of stage manipulators.

Monochromatic aluminum K-alpha X-rays are normally produced by diffracting and focusing a beam of non-monochromatic X-rays off of a thin disc of natural, crystalline quartz with a <1010> lattice. The resulting wavelength is 8.3386 angstroms (0.83386 nm) which corresponds to a photon energy of 1486.7 eV. The energy width of the monochromated X-rays is 0.16 eV, but the common electron energy analyzer (spectrometer) produces an ultimate energy resolution on the order of 0.25 eV which, in effect, is the ultimate energy resolution of most commercial systems. When working under everyday conditions, the typical high energy resolution (FWHM) is usually 0.4-0.6 eV.

Non-monochromatic magnesium X-rays have a wavelength of 9.89 angstroms (0.989 nm) which corresponds to a photon energy of 1253 eV. The energy width of the non-monochromated X-ray is roughly 0.70 eV, which, in effect is the ultimate energy resolution of a system using non-monochromatic X-rays. Non-monochromatic X-ray sources do not diffract out the other nearby X-ray energies and also allow the full range of high energy Bremsstrahlung X-rays (1–12 keV) to reach the surface. The typical ultimate high energy resolution (FWHM) for this source is 0.9–1.0 eV, which includes with the spectrometer-induced broadening, pass-energy settings and the peak-width of the non-monochromatic magnesium X-ray source.

Uses and capabilities

XPS is routinely used to determine:

what elements and the quantity of those elements that are present within ~10 nm of the sample surface
what contamination, if any, exists in the surface or the bulk of the sample
empirical formula of a material that is free of excessive surface contamination
the chemical state identification of one or more of the elements in the sample
the binding energy (BE) of one or more electronic states
the thickness of one or more thin layers (1–8 nm) of different materials within the top 10 nm of the surface
the density of electronic states

Capabilities of advanced systems

measure uniformity of elemental composition across the top the surface (aka, line profiling or mapping)
measure uniformity of elemental composition as a function of depth by ion beam etching (aka, depth profiling)
measure uniformity of elemental composition as a function of depth by tilting the sample (aka, angle resolved XPS)

Industries that use XPS

Adhesion, Agriculture, Automotive, Battery, Beverage, Biotech, Canning, Catalyst, Ceramic, Chemical, Computer, Cosmetic, Electronics, Environmental, Fabrics, Food, Fuel cells, Geology, Glass, Laser, Lighting, Lubrication, Magnetic memory, Mineralogy, Mining, Nuclear, Packaging, Paper and wood, Plating, Polymer and plastic, Printing, Recording, Semiconductor, Steel, Textiles, Thin-film coating, Welding

Routine limits of XPS

Quantitative accuracy

Quantitative accuracy depends on several parameters such as: S/N, peak intensity, accuracy of relative sensitivity factors, correction for electron transmission function, surface volume homogeneity, correction for energy dependency of electron mean free path, and degree of sample degradation due to analysis.
Under optimum conditions, the quantitative accuracy of the atom % values calculated from the Major XPS Peaks is 90-95% of the atom % values of each major peak. If a high level quality control protocol is used, the accuracy can be further improved.
Under routine work conditions, where the surface is a mixture of contamination and expected material, the accuracy ranges from 80-90% of the value reported in atom % values.
The quantitative accuracy for the weaker XPS signals, that have peak intensities 10-20% of the strongest signal, are 60-80% of the true value.

Analysis times

1–10 minutes for a survey scan that measures the amount of all elements, 1–10 minutes for high energy resolution scans that reveal chemical state differences, 1–4 hours for a depth profile that measures 4–5 elements as a function of etched depth (usual final depth is 1,000 nm)

Detection limits

0.1–1.0 atom % (0.1 atom% = 1 part per thousand = 1,000 ppm). Ultimate detection limit for most elements is approximately 100 ppm, which requires 8–16 hours.)

Analysis area limits

Analysis area depends on instrument design. The minimum analysis area ranges from 10 to 200 micrometres. Largest size for a monochromatic beam of X-rays is 1–5 mm. Non-monochromatic beams are 10–50 mm in diameter. Spectroscopic image resolution levels of 200 nm or below has been achieved on latest imaging XPS instruments using synchrotron radiation as X-ray souce.

Sample size limits

Older instruments accept samples: 1x1 to 3x3 cm. Very recent systems can accept full 300 mm wafers and samples that are 30x30 cm.

Degradation during analysis

Depends on the sensitivity of the material to the wavelength of X-rays used, the total dose of the X-rays, the temperature of the surface and the level of the vacuum. Metals, alloys, ceramics and most glasses are not measurably degraded by either non-monochromatic or monochromatic X-rays. Some, but not all, polymers, catalysts, certain highly oxygenated compounds, various inorganic compounds and fine organics are degraded by either monochromatic or non-monochromatic X-ray sources.
Non-monochromatic X-ray sources produce a significant amount of high energy Bremsstrahlung X-rays (1–15 keV of energy) which directly degrade the surface chemistry of various materials. Non-monochromatic X-ray sources also produce a significant amount of heat (100 to 200 °C) on the surface of the sample because the anode that produces the X-rays is typically only 1 to 5 cm away from the sample. This level of heat, when combined with the Bremsstrahlung X-rays, acts synergistically to increase the amount and rate of degradation for certain materials. Monochromatic X-ray sources, because they are far away (50–100 cm) from the sample, do not produce any heat effects. Monochromatic X-ray sources are monochromatic because the quartz monochromator system diffracted the Bremsstrahlung X-rays out of the X-ray beam which means the sample only sees one X-ray energy, for example: 1.486 keV if aluminum K-alpha X-rays are used.
Because the vacuum removes various gases (eg O₂, CO) and liquids (eg water, alcohol, solvents) that were initially trapped within or on the surface of the sample, the chemistry and morphology of the surface will continue to change until the surface achieves a steady state. This type of degradation is sometimes difficult to detect.

Materials routinely analyzed by XPS

Inorganic compounds, metal alloys, semiconductors, polymers, pure elements, catalysts, glasses, ceramics, paints, papers, inks, woods, plant parts, make-up, teeth, bones, human implants, bio-materials, viscous oils, glues, ion modified materials

Organic chemicals are not routinely analyzed by XPS because they are readily degraded by the either the energy of the X-rays or the heat from non-monochromatic X-ray sources.

Analysis details

Charge compensation techniques

Low Voltage Electron Beam (1-20 eV) [aka Electron Flood Gun]
UV light
Low Voltage Argon Ion Beam with Low Voltage Electron Beam (1-10 eV)
Aperture mask
Mesh screen with Low Voltage Electron Beam

Sample preparation

Sample handling
Sample cleaning
Sample mounting

Data processing

Charge referencing insulators

Guidelines
Using C 1s BE of hydrocarbon components (moieties)

Peak-fitting

Guidelines
Typical FWHM

Related methods

UPS, Ultra-violet photoelectron spectroscopy (for gases, aka PES)
PES, Photoemission spectroscopy (for solid surfaces, aka UPS)
ZEKE, Zero Electron Kinetic Energy spectroscopy
AES, Auger electron spectroscopy (AES)